Corrosion is defined as the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.
Corrosive attack begins on the surface of the metal. The corrosion process involves two chemical changes. The metal that is attacked or oxidized undergoes an anodic change, with the corrosive agent being reduced and undergoing a cathodic change. The tendency of most metals to corrode creates one of the major problems in the maintenance of aircraft, particularly in areas where adverse environmental or weather conditions exist.
Chromium-based anti-corrosive systems containing hexavalent chromium compounds have proven to be an extremely useful and versatile group of chemistries that are extensively used in aircraft metal treatment processes. They impart many beneficial and essential anti-corrosive characteristics to metallic substrates on which they are applied and have been used extensively for the pre-treatment of metals before coating, adhesive bonding and surface finishing.
Chemically, chromium-based anti-corrosive systems have involved the combination(s) of hexavalent chromium (e.g., CrO3, CrO42−, Cr2O72−) and hydrofluoric acid (HF) in the case of aluminum and its alloys. The hydrofluoric acid removes oxide film from the surface of the metallic substrate (e.g., aluminum) and the hexavalent chromium reacts with the exposed metal and a trivalent chromium oxide precipitates. Using aluminum as an example:Cr2O72−+2Al0+2H+→Cr2O3.H2O+Al2O3 
Chromium oxide such as that produced according to the above reaction is quite useful in anti-corrosive applications. It is quite stable in alkaline environments, it is water repellant (hydrophobic) and may act as a barrier coating towards water. Finally, it exhibits a “self-healing effect,” that is, residual hexavalent chromium in the coating may react with damaged areas of the coating, thereby producing more trivalent chromium oxide at damaged sites and therefore “healing” itself.
Consequently, chromium-based, and in particular hexavalent chromium-based systems, have been extensively used in the aircraft industry because they have proven to be: highly effective at preventing corrosion and as an adhesion promoter for organic coatings and adhesives; particularly resilient as the application/treatment process exhibits a low sensitivity towards variation in process conditions; extremely effective on most/all aluminum alloys; and ensure considerable quality control characteristics as a skilled worker may tell the amount of chromium on the surface of a substrate by mere inspection (color) of the coating.
Concern about chromium, in particular hexavalent chromium, in the environment has generated a need to replace chromium-based systems. Hexavalent chromium salts are classified as hazardous substances (toxic, sensitizing and carcinogenic). Consequently, they are environmentally and toxicologically undesirable. The European Parliament has published directives requiring the elimination of hexavalent chromium such as directive 2002/95/EC for electrical and electronic equipment and directive 2000/53/EC for the automotive sector. Therefore “environmentally friendly,” commercially acceptable alternative to chromium-based systems are highly desirable.
Processes and compositions for Cr-free conversion coatings with good corrosion resistance were described in the Patent Application US2010/0009083 A1 by the same inventors. This U.S. Patent Application describes a conversion coating for the treatment of surfaces comprising a conducting polymer dispersion containing an inorganic metallic salt of at least one of molybdenum, magnesium, zirconium, titanium, vanadium, cerium, hafnium, silicon, aluminum, boron, cobalt and zinc. The conversion coatings described in the abovementioned patent application showed very good corrosion performance. However, a good adhesion of the coatings to subsequent organic coatings was not achieved without jeopardizing the corrosion protection.
A significant improvement was developed by the same inventors and a new patent application was filed in October 2012, U.S. patent application Ser. No. 13/662,412 (published as US2013/052352 A1). This patent application describes a conversion coating comprising a conducting polymer dispersion, at least one silane and inorganic salts selected from at least one of molybdenum, magnesium, zirconium, titanium, vanadium, cerium, hafnium, silicon, aluminum, boron, cobalt and zinc. The silane compounds enhanced the adhesion performance so the coating complied with the requirements of aeronautical applications, while the corrosion protection as described in the examples of US2010/0009083 A1 was maintained. Moreover, the new conversion coatings obtained with the conducting polymer dispersions comprising at least one silane compound offered low surface contact electrical resistance, compliant with the requirements for aeronautical applications.
Now, the challenge is to optimize the chemical conversion process and the conversion coating to further enhance the corrosion protection without jeopardizing the adhesion.
Lanthanide ions as Ce3+ and Ce4+ forming insoluble hydroxides show low toxicity and are economically competitive products, since cerium is relatively abundant in nature. Therefore, cerium has been investigated to develop corrosion protection systems for aluminum alloys as alternative to chromates.
In particular, M. Bethencourt et al., in a paper titled “High protective, environmental friendly and short-time developed conversion coatings for aluminum alloys”, which was published in Applied Surface Science 189 (2002) 162-173, described conversion coating treatments for AA5083 based on Ce(NO3)2 and CeCl3 0.005M solutions with pH adjusted to 5.5, deposited at temperatures ranging from 298K to 363K, and immersion times between 0.08 h and 24 h. The corrosion performance was studied through linear polarization curves. The best results are reported for Ce(NO3)2 treatments carried out at 348K for 120 min, showing an increase of the polarization resistance by a factor higher than 12.000 comparing to the bare alloy.
The above-mentioned publication deals with cerium conversion treatments containing no zirconium salts, conducting polymers or silanes. Further, the process described in M. Bethencourt et al. involves high temperatures and long immersion times to obtain satisfactory corrosion protection results, different from the process of the present invention wherein the coating may be carried out without heat treatment, preferably at room temperature between 1 to 10 minutes.
Botana et al., in the patent PCT WO2004/059035 A1 “Method of obtaining chromate-free conversion coatings on aluminum alloys” (Universidad de Cádiz, 2004) discloses a coating consisting of a mix of cerium rich islands onto intermetallic particles and aluminum oxide layer in the alloy matrix. Such a coating is obtained with a dipping process in an aerated solution containing 0.001-0.01M Ce(NO3)3 or 0.001-0.01M CeCl3. The process is carried out in a temperature range of 323-363K for a maximum immersion time of 120 min. In one of the described options 0.5-30 mL/L H2O2 is added to the already heated solution, and immersion time is shortened to 30 minutes. The treated panels passed 168 h of salt spray test under ASTM B-117 without signs of corrosion. In electrochemical tests, a factor of improvement of 40 in the polarization resistance is obtained with respect to the panels treated at room temperature.
However, patent application WO2004/059035 A1 is related to cerium conversion coatings without the presence of conducting polymers or silanes. Furthermore, the procedure described in WO2004/059035 A1 involves high temperature and/or long immersion times comparing with the process for treatment of a metallic surface of the present application.
C. Rosero-Navarro et al., in the PCT patent application WO2011/058209 A1 “Vitreous coatings made using the sol-gel process for protecting metals against corrosion” (Consejo Superior de Investigaciones Científicas (CSIC), 2011), describe a composition and methods to obtain a vitreous coating made by sol-gel process. Said vitreous coating contains Ce3+ ions in its structure, which migrates to damaged regions when the metal suffers corrosion phenomena. The hybrid coatings, formed from solutions containing cerium salts and organic complexing agent, are sintered in temperatures up to 250° C., and the resulting coating shows a thickness in the range of 100-1000 nm. The cerium salt is selected from the group of chlorides, sulfate, nitrates or halide, being preferably nitrate. The complexing agents are acetyl-acetone, glacial acetic acid, citric acid, diethanol-amine, or other compounds with carboxylic groups, and improve the stability of the sol and allows controlling of the final pH. One of the examples deals with the treatment of AA2024 panels (see composition in Table 1), coatings with a sol composed of Ce(NO3)36H2O, glacial acetic acid, citric acid and butanodiol, with a final pH value of 2. The coating is sintered during 12 hours at 120° C., and it is further coated with an epoxy primer. The coated panel was exposed to salt spray test (ASTM B-117) with a scratch, and after 1000 h of exposure the scratch was still protected, attributed by the inventors to the passivation and active protection of the coating.
Another sol-gel process and coating for aluminum alloys is disclosed by F. Ansart et al. in the PCT patent application WO2013/054064 A1 “Process for the anticorrosion treatment of a solid metal substrate and treated solid metal substrate capable of being obtained by such a process” (Université Paul Sabatier Toulouse III, 2013). The described hybrid coating containing cerium nitrate, offers high adherence to the substrate and high mechanical resistance. Its corrosion protection performance is due to the barrier effect and self-healing or active corrosion protection in corrosion pits or other damaged areas. The active protection is conferred by the cerium compound, which can be either chloride, nitrate, acetate or sulfate. A particular coating is described in an example, composed of 3-(glycidoxypropyl)-trimehoxysilane (GPTMS), aluminum tri(s-butoxyde) (ASB) and cerium nitrate, with final cerium content of 0.01 mol/L. The sol is deposited onto AA2024-T3 (see composition in Table 1) and sintered at 110° C. for 3 h. The resulting coating shows a thickness of 6 microns, and offers an exposition between 96 and 800 h in salt spray test without corrosion.
The patent application US2012/0204762 A1 “Aqueous silane systems for bare corrosion protection and corrosion protection of metals” (P. Albert et al., Evonik Degussa GmbH, 2012) relates to an aqueous silane-based composition to be used in corrosion protection in metals. The composition contains metal salts of cerium (III) or cerium (IV) between other metals, and particular preference is given to nitrates and acetates. The resulting coating prevents pitting corrosion in aluminum alloys.
Patent applications WO2011/058209 A1 and WO2013/054064 A1, above summarized, describe sol-gel coatings incorporating cerium compounds. The compositions of both inventions are based on organic solvents, and require thermal treatment at high temperature as the final step of the coating process. Similar characteristics are shown by the silane system presented in the patent application US2012/0204762 A1.
The U.S. Pat. No. 6,077,885 “Chromate-free protective coatings” (H. E. Hager et al., The Boeing Company, 2000) discloses a polymeric coating providing corrosion protection of aluminum and its alloys used in the aircraft industry. The coatings of the invention have a “site blocking” or “buffering” action in that the corrosion inhibitors of the coatings are mobile and migrate into damaged areas to protect them from corrosion. This mobility is a result of the solubility of the inhibitors in the polymer matrix. The coating consists of a film-forming organic polymer or sol-gel containing metallic salts including cerium oxalates, acetates, borate, chlorides and others. The coating may be applied to aluminum alloy substrate as primer coat, a pigmented coat, or as a unicoat. The coating is preferably prepared in liquid form, with the polymer dispersed or dissolved, and the salts present controlled solubility in the polymer or sol-gel, or either they should be suspended in the blend. The salt content is preferably in the range of 100-300 ppm. The polymers are epoxy based, polyimides, polyurethanes, acrylics and alkyd-based systems. 2024-T3 panels (see composition in Table 1) coated with a formulations containing cerium oxalate and/or cerium acetate as inhibitor showed excellent dry and wet adhesion, but poor corrosion results in 1500 h and 3000 h of 5 wt. % NaCl salt spray test on described panels. In the organic polymer or sol-gel coating containing cerium compounds described in this patent, good adhesion results are reported for the cerium salt containing formulation, but with poor corrosion results in salt spray tests.
H. Shoji et al., in the U.S. Pat. No. 6,190,780 “Surface treated metal material and surface treating agent” (Nippon Steel Corporation, 2001), disclose a corrosion protection treatment for metal surfaces including aluminum and its alloys, composed mainly of a oxyacid compound of a rare earth element, other inorganic or organic compounds of rare earth elements, a resin, and an organic corrosion inhibitor. The oxyacid anion may be a phosphate, tungstate, molybdate and/or vanadate, the rare earth element can be cerium, and the corrosion inhibitor a conducting polymer. The treatment shows good adhesion and corrosion performance in the examples comprising the coating of various steel sheets. Although the corrosion protection treatment disclosed in this patent may comprise using cerium and conducting polymer as corrosion inhibitor, silanes are not mentioned. Adhesion and corrosion results are only depicted for zinc and steel.
The U.S. Pat. No. 6,875,479 “Method for coating metal surfaces with aqueous, polymer-containing composition, said aqueous composition and the use of the coated substrates” (C. Jung et al., Chemetall GmbH, 2005) relates to a method for coating a metallic surface including aluminum, with an aqueous composition that contains an polymeric film-forming agent, fine inorganic particles within the range from 0.005 μm to 0.3 μm, at least one organic corrosion inhibitor, optionally a silane, and other optional compounds. One of the proposed compounds to add in fine particle form is cerium dioxide, whereas conducting polymers are mentioned as possible organic corrosion inhibitor. The examples are focused on the treatment of galvanized steel sheets, obtaining films with thickness in the range of 0.8-1 μm, and showing no corrosion up to 720 hours salt spray test under ASTM B-117. Therefore, the composition described in this patent may contain a conducting polymer as corrosion inhibitor, cerium compounds and optionally a silane. However, different to the inorganic salts used in our invention, the cerium is added as cerium dioxide in the form of fine particles. Moreover, the formulation described in U.S. Pat. No. 6,875,479 contains also a film forming organic polymer, and a thermal treatment is required according to its curing conditions. Adhesion and corrosion results are only depicted for galvanized steel sheets.